1. Field of the Invention
The present invention relates generally to electronic circuits and, in particular, to a bipolar charge pump switching circuit that transfers the maximum possible charge to the charge pump capacitor while retaining a cascode configuration for high breakdown.
2. Discussion of the Prior Art
Electronic systems often include circuits that require an operating voltage higher than the available supply. For example, some circuits in an automotive system require an input voltage that is higher than the 12 Volts available from a standard automobile battery. In these applications, a dedicated circuit is needed to "pump" the supply to the desired voltage level.
A simple schematic of a conventional charge pump circuit 10 is shown in FIG. 1. The circuit 10 consists, essentially, of a charge pump capacitor 12 that has one of its plates connected to the supply V+, shown as 12V in FIG. 1, and its other plate connectable to either ground or to the supply by switch 14. Initially, the switch 14 is in Position 1 which connects the bottom plate of the capacitor 12 to ground, causing the top plate of capacitor 12 to charge to the supply, i.e., 12V in the FIG. 1 example. Actually, the top plate of capacitor 12 may charge to slightly less than supply because of the 1 V.sub.BE drop across diode 16. After the top plate of capacitor 12 is fully charged, switch 14 is repositioned to Position 2, connecting the bottom plate of capacitor 12 to the supply. With the bottom plate of capacitor 12 now connected to the supply, the potential of the top plate increases by an additional V+. That is, nearly twice the supply is now available to be drawn upon by load 18 as needed, with diode 16 providing isolation for capacitor 12.
Typically, switch 14 is implemented in so-called MOS (metal-oxide-semiconductor) technology which provides crisp on-off switching capability and rail-to-rail voltage swing for charging the pump capacitor 12.
One simple and widely implemented CMOS (complimentary-metal-oxide-semiconductor) circuit configuration utilized in realizing switch 14 is the inverter 20 shown in FIG. 2. A square wave input to the gates of n-channel transistor 22 and p-channel transistor 24 causes the transistors 22 and 24 to alternately turn on to provide the switching action described above in conjunction with FIG. 1. The period of the square wave input may be chosen to provide a charging frequency for capacitor 12 sufficient to satisfy the demands of the load 18.
However, a major deficiency of these MOS switches is that, in some harsh operating environments, the breakdown voltages of the MOS transistors are insufficient for the transistors to withstand the high voltage transients. For example, in automotive systems, the possibility of inadvertent reverse battery conditions or loose battery cables has caused some automobile manufacturers to specify that integrated circuits utilized in these applications be capable of standing off up to 60V, well above the breakdown voltage of commonly available MOS transistors. Thus, additional protective circuitry is required to shield the MOS switching transistors from the transient voltage levels.
Lateral PNP bipolar transistors, as commonly constructed utilizing conventional integrated circuit fabrication processes, are rugged enough to withstand the high transients described above, and when combined with complementary vertical NPN bipolar transistors form a desirable switching technology for charge pumps.
While NPN transistors typically cannot stand off sufficiently high voltages individually, it is well known that high breakdown NPN structures can be obtained by stacking the NPN transistors in a so-called cascode configuration. NPN transistors tend to easily saturate, however, causing them to turn off slowly. This is inconsistent with the need for the high switching speeds required to maintain the desired charge level on the pump capacitor, which typically can only be as large as 20-40 pF given present integrated circuit fabrication technology. Therefore, to realize the high switching speeds, the NPN transistors must be kept out of saturation.